Method for manufacturing a semiconductor device

ABSTRACT

A method for fabricating a semiconductor device includes: forming a trench in a first major surface of a semiconductor body having a first conductivity type; forming a gate in the trench; forming a body region of a second conductivity type in the semiconductor body; implanting a second dopant species into a first region of the body region and a first dopant species into a second region of the body region, the first dopant species providing the first conductivity type, the second dopant species being different from the first dopant species and reducing the diffusion of the first dopant species in the semiconductor body; and thermally annealing the semiconductor body to form a source region that includes the first and second dopant species, and to produce a pn-junction between the source and body regions at a depth dpn from the first major surface, wherein 50 nm&lt;dpn&lt;300 nm.

BACKGROUND

Semiconductor devices, such as transistor devices, may be used forvarious applications, such as switches in power supplies and powerconverters. Examples of transistor devices include a Metal OxideSemiconductor Field Effect Transistor (MOSFETs) and an Insulated GateBipolar Transistor (IGBT). It is desirable that the semiconductordevices, including transistor devices have a good and reliableperformance.

SUMMARY

In an embodiment, a semiconductor device is provided. The semiconductordevice comprises a semiconductor body comprising a first major surfaceand a second major surface opposing the first major surface wherein thesemiconductor body comprises a first conductivity type, a gate arrangedin a trench extending into the semiconductor body from the first majorsurface, a body region comprising a second conductivity type, the secondconductivity type opposing the first conductivity type; a source regioncomprising the first conductivity type, the source region being arrangedon the body region and comprising at least one first dopant speciesproviding the first conductivity type and at least one second dopantspecies that is different from the first dopant species, the seconddopant species affecting the diffusion of the first dopant species inthe semiconductor body, wherein the source region forms a pn-junctionwith the body junction, the pn-junction being arranged at a depth d_(pn)from the first major surface, wherein 50 nm<d_(pn)<300 nm, and a drainregion comprising the first conductivity type, the drain region beingarranged in the semiconductor body under the trench.

In an embodiment, a method for fabricating a semiconductor device isprovided. The method comprises forming a trench in a first major surfaceof a semiconductor body comprising a first conductivity type, forming agate in the trench, forming a body region of a second conductivity typein the semiconductor body, implanting a first dopant species into afirst region of the body region, implanting a second dopant species intoa second region of the body region, wherein the first dopant speciesprovides the first conductivity type and wherein the second dopantspecies is different from the first dopant species and affects thediffusion of the first dopant species in the semiconductor body,thermally annealing the semiconductor body, and forming a source region,the source region comprising phosphorous and carbon, and producing apn-junction between the source region and the body region at a depthd_(pn) from the first major surface, wherein 50 nm<d_(pn)<300 nm.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The elements of the drawings are not necessarily to scale relative toeach other. Like reference numerals designate corresponding similarparts. The features of the various illustrated embodiments can becombined unless they exclude each other. Exemplary embodiments aredepicted in the drawings and are detailed in the description whichfollows.

FIG. 1 illustrates a cross-sectional view of a portion of asemiconductor device according to an embodiment.

FIG. 2 illustrates a flow diagram of a method for fabricating asemiconductor device according to an embodiment.

FIG. 3 illustrates a cross-sectional view of a portion of asemiconductor device according to an embodiment.

FIG. 4 illustrates a cross-sectional view of a portion of asemiconductor device according to an embodiment.

FIG. 5 illustrates a cross-sectional view of a portion of asemiconductor device according to an embodiment.

FIG. 6 illustrates a cross-sectional view of a portion of asemiconductor device according to an embodiment.

FIG. 7 illustrates a cross-sectional view of a portion of asemiconductor device according to an embodiment.

FIG. 8 illustrates a cross-sectional view of a portion of asemiconductor device according to an embodiment.

FIG. 9 illustrates a cross-sectional view of a portion of asemiconductor device according to an embodiment.

FIG. 10 illustrates a cross-sectional view of a portion of asemiconductor device according to an embodiment.

FIG. 11 illustrates a cross-sectional view of a portion of asemiconductor device according to an embodiment.

FIG. 12 illustrates a cross-sectional view of a portion of asemiconductor device according to an embodiment.

FIG. 13 illustrates a cross-sectional view of a portion of asemiconductor device according to an embodiment.

FIG. 14 illustrates a cross-sectional view of a portion of asemiconductor device according to an embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top”,“bottom”, “front”, “back”, “leading”, “trailing”, etc., is used withreference to the orientation of the figure(s) being described. Becausecomponents of the embodiments can be positioned in a number of differentorientations, the directional terminology is used for purposes ofillustration and is in no way limiting. It is to be understood thatother embodiments may be utilized and structural or logical changes maybe made without departing from the scope of the present invention. Thefollowing detailed description, thereof, is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

A number of exemplary embodiments will be explained below. In this case,identical structural features are identified by identical or similarreference symbols in the figures. In the context of the presentdescription, “lateral” or “lateral direction” should be understood tomean a direction or extent that runs generally parallel to the lateralextent of a semiconductor material or semiconductor carrier. The lateraldirection thus extends generally parallel to these surfaces or sides. Incontrast thereto, the term “vertical” or “vertical direction” isunderstood to mean a direction that runs generally perpendicular tothese surfaces or sides and thus to the lateral direction. The verticaldirection therefore runs in the thickness direction of the semiconductormaterial or semiconductor carrier.

As employed in this specification, when an element such as a layer,region or substrate is referred to as being “on” or extending “onto”another element, it can be directly on or extend directly onto the otherelement or intervening elements may also be present. In contrast, whenan element is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present.

As employed in this specification, when an element is referred to asbeing “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

The Figures illustrate relative doping concentrations by indicating “−”or “+” next to the doping type “n” or “p”. For example, “n−” means adoping concentration which is lower than the doping concentration of an“n”-doping region while an “n+”-doping region has a higher dopingconcentration than an “n”-doping region. Doping regions of the samerelative doping concentration do not necessarily have the same absolutedoping concentration. For example, two different “n”-doping regions mayhave the same or different absolute doping concentrations.

Discrete power switches such as power MOSFET devices with blockingvoltages of up to 300V, 100V, 60V, or 30V, for example, may have therequirement to provide ultra-low source path resistances. The source ofMOSFETs is often built up by high dose As (arsenic) implantation due tothe shallow profiles created by As Implantation and a low diffusivityduring a thermal budget. Furthermore, As ions have a high stopping powerin semiconductor material like e.g. Silicon, providing a lowimplantation depth at a given implantation energy which is compatiblewith commonly used beamline implantation tools in productiveenvironment. Alternatively, As ions exhibit steeper doping gradientswhen implanted to a predefined depth into the semiconductor material ascompared to lighter ions as e. g. Phosphorous. All this enables steepdoping profiles which are favorable for the Source doping of a powerMOSFET without any additional effort, i.e. cost, in the manufacturingprocess by selecting As as the dopant ion. However, with high dose AsImplantation, there is a risk of vacancy formation from As precipitationwhich potentially causes drain-source leakage currents (IDSS), which mayin turn result in chip failure and/or low production yields.

According to embodiments described herein, the high dose As Implantationis replaced by a combination of Carbon Implantation and PhosphorousImplantation in order to achieve the same (or even better) electricalcharacteristics while reducing or even eliminating the risk of IDSSfailure due to the creation of As defects.

In an embodiment, a semiconductor device 20 comprises a semiconductorbody 21 comprising a first major surface 22 and a second major surface23 opposing the first major surface 22. The semiconductor body 21includes a first conductivity type. The semiconductor device 20 furthercomprises a gate 24 arranged in a trench 25 that extends into thesemiconductor body 21 from the first major surface 22. The semiconductordevice 20 includes a body region 26 comprising a second conductivitytype which opposes the first conductivity type. For example, if thefirst conductivity type is n-type, the second conductivity type isp-type and if the first conductivity type is p-type, the secondconductivity type is n-type. The semiconductor device 20 furtherincludes a source region 27 comprising the first conductivity type thatis arranged on the body region 26 and a drain region 28 comprising thefirst conductivity type which is arranged in the semiconductor body 21under the trench 25.

The source region 27 is arranged on the body region 26 such that a pnjunction is formed at the interface 29 between the source region 27 andthe body region 26. The interface 29 and the pn junction is arranged ata depth d_(pn) from the first major surface 22. At least a portion ofthe interface and at least a portion of the pn junction may lie at adepth d_(pn) in the range of 50 nm to 300 nm.

The source region 27 comprises two or more different dopants or dopantspecies. At least one of the dopants or dopant species provides donors,in embodiments in which the source region 27 is n-type, or acceptors, inembodiments in which the source region 27 is p-type. One or more of thedopants or dopant species comprises an element or ion which differs fromthe donor or acceptor and which is selected to affect the diffusion ofthe donor or acceptor in the semiconductor body, in particular diffusionof the donor or acceptor in the semiconductor body upon application ofheat to the semiconductor body, e.g. upon thermal annealing. One or morefirst dopant species is used to provide conductivity in the sourceregion 27 and one or more second dopant species, that are different tothe first dopant species, are used to control the volume of the sourceregion 27. The second dopant species may be considered to have adiffusion reduction or deceleration function when present in combinationwith the first dopant species, the first dopant species providing donorsor acceptors and conductivity in the source region 27.

In some embodiments, the source region 27 comprises phosphorus as afirst dopant species which acts as a donor and carbon as a second dopantspecies which affects the diffusion of phosphorus within thesemiconductor body 21 upon the application of heat to the semiconductorbody 21. The source region 27 may further comprise one or more offluorine, antimony, or germanium as a second dopant species. The sourceregion 27 is free of implanted arsenic dopants and may be entirelyarsenic free.

The semiconductor device 20 includes a transistor structure 30 which istypically provided as one of a large number of substantially identicaltransistor structures or transistor cells which are coupled together. Insome embodiments, the transistor structures or transistor cells arecoupled together such that the semiconductor device provides a singleswitch. However, groups of transistor structures or transistor cells maybe coupled together such that the semiconductor device 20 provides twoor more switches. The semiconductor device may provide a transistordevice having a blocking voltage of up to 300 V and, therefore, differsfrom a logic device not only in this blocking voltage, but also in thedepth or spacing of the pn junction between the source region 27 andbody region 26 from the first major surface 22. At least a portion ofthe pn junction between the source region 27 and body region 26 isarranged at a depth d_(pn) from the first major surface 22, where d_(pn)lies in the range of 50 nm to 300 nm.

The source region 27 may have the form of a well which borders on thefirst major surface 22 and the body region 26 may surround the sourceregion 27 such that the source region 27 is not in direct contact withthe semiconductor body 21.

The trench 25 may comprise an insulating material which lines thesidewalls and base of the trench 25 and electrically isolates the gate24 positioned within the trench 25 from the semiconductor body 21. Thegate 24 may comprise conductive material and may comprise polysilicon ormetal.

The concentration of the first dopant species, for example phosphorus,in the source region 27 may be in the range of 5.10¹⁹ atoms/cm³ up to7.10²⁰ atoms/cm³ or in the range of 1.10²⁰ atoms/cm³ up to 3.10²⁰atoms/cm³ and the concentration of the second dopant species, forexample carbon, in the source region 27 may be in the range of 1.10¹⁹ upto 7.10²⁰ atoms/cm³. The distribution of the first dopant speciesconcentration and/or distribution of the second dopant speciesconcentration within the source region 27 may vary, for example bespatially non-uniform.

In embodiments in which the source region includes phosphorous andcarbon, the distribution of the phosphorus concentration and/ordistribution of the carbon concentration within the source region 27 mayvary, for example be spatially non-uniform. For example, theconcentration of carbon may be higher towards the base of the sourceregion 27 than towards the top of the source region 27, that is higherin directions towards the interface 29 between the source region 27 andthe body region 26 and lower in directions towards the first majorsurface 22. For example, the concentration of carbon may even reach itsmaximum not inside the source region 27 but inside the body region 26.

In some embodiments, the source region 27 extends from a sidewall 31 ofthe trench 25 and the body region 26 extends from the sidewall 31 of thetrench 25 such that the interface 29 between the source region 27 andthe body region 26 extends from the sidewall 31 to the first majorsurface 22.

A source region comprising two or more differing dopants elements andhaving a pn junction between the source region and the body region at adepth d_(pn) from the first major surface of 50 nm to 300 nm may be usedin semiconductor devices including differing transistor structures andis not limited to the exemplary transistor structure 30 illustrated inFIG. 1 . For example, the semiconductor device 20 may further include abody contact region comprising the second conductivity type which ismore highly doped than the body region. The semiconductor device mayinclude a further trench which extends into the semiconductor body fromthe first major surface. The semiconductor device may further include afield plate which is arranged in the same trench as the gate and underthe gate or in a separate trench. The trenches may be elongate, forexample strip-like in plan view, or columnar. The semiconductor body 21may include an epitaxial layer, for example a monocrystalline epitaxialsilicon layer, or a single-crystal substrate, for example silicon wafer.

FIG. 2 illustrates a flow diagram 40 of a method for fabricating asemiconductor device. In box 41, a trench is formed in a first surfaceof a semiconductor body which comprises a first conductivity type. Inbox 42, a gate is formed in the trench. In box 43, a body region of thesecond conductivity type is formed in the semiconductor body. In box 44,one or more second dopant species, for example carbon and optionallyfluorine, is implanted into a first region of the semiconductor of thebody region. In box 45, one or more first dopant species, for examplephosphorus ions, is implanted into a second region of the body region.In box 46, the semiconductor body is thermally annealed forming thesource region comprising the first and second dopant species, forexample phosphorus and carbon, and producing a pn junction between thesource region and the body region at a depth d_(pn) from the first majorsurface, whereby d_(pn) lies in the range of 50 nm to 300 nm, forexample.

The first and second dopant species may be implanted sequentially orconcurrently into the body region. For example, the second dopantspecies may be implanted into a first region of the body region and,subsequently, the first dopant species may be implanted into a secondregion of the body region. In other embodiments, the order ofimplantation can be reversed, such that the first dopant species isfirst implanted and subsequently, the second dopant species is implantedinto the body region.

In some embodiments, the carbon and phosphorus ions may be implantedsequentially or concurrently into the body region. For example, thecarbon may be implanted into a first region of the body region and,subsequently, phosphorus ions may be implanted into a second region ofthe body region. In other embodiments, the order of implantation can bereversed, such that the phosphorus ions first implanted andsubsequently, the carbon is implanted into the body region.

The first region and the second region may refer to the same volume ormay refer to overlapping volumes or may refer to different volumes ofthe body region. For example, the first dopant species may be implantedinto at least a portion of the first region which already includes thesecond dopant species. This portion of the first region forms the secondregion. For example, phosphorus ions may be implanted into at least aportion of the first region which already includes carbon.

The concentration of the first and second dopant species within thesource region may vary and be spatially non-uniform. For example, theconcentration of the second dopant species may be greater in a directiontowards the interface between the source region and the body region. Insome embodiments, the first dopant species and/or the second dopantspecies are each implanted in multiple steps. The implantationconditions may be for each step may be carried out such that the firstdopant species and/or the second dopant species are implanted atdecreasing or increasing depth from the first major surface, forexample.

The concentration of phosphorus and/or carbon within the source regionmay vary and be spatially non-uniform. For example, the concentration ofcarbon may be greater in a direction towards the interface between thesource region and the body region. In some embodiments, the phosphorusions and/or the carbon are each implanted in multiple steps. Theimplantation conditions for each step may be carried out such that thephosphorus ions and/or carbon are implanted at decreasing or increasingdepth from the first major surface, for example.

The conditions of the thermal annealing may be selected depending on thefirst and second dopant species. The thermal annealing may be carriedout at a temperature in the range of 900° C. to 1050° C. for a time inthe range of 1 s (second) to 360 s (seconds). In some embodiments, amultiple stage thermal annealing is carried out. For example, threeannealing treatments, each for a time period of 30 seconds, may becarried out at a temperature between 950° C. and 1000° C. Theseconditions may be used for phosphorous and carbon, for example.

The method according to any one of the embodiments described herein isused to form a source region of a transistor device. As discussed above,a transistor device typically includes a large number of transistorstructures or cells which are coupled in parallel to produce a singleswitch. The source including at least two different dopant species, suchas carbon and phosphorus, may be used in a various types of transistorstructure which comprise differing features. The method for fabricatingthe semiconductor device and transistor structure may also, therefore,include additional embodiments for forming various transistorstructures.

In an embodiment, the method further comprises forming a field plate ina base of the trench and forming the gate on the field plate. The gateand the field plate are arranged in a single trench and are electricallyinsulated from one another and from the semiconductor body. The fieldplate may include poly silicon or a metal and the gate may also includepoly silicon or metal. The method may further comprise forming aninsulating material on walls of the trench and forming a field plate onthe insulating material, forming a further insulating material onsidewalls of the trench and on the field plate and forming the gate onthe further insulating layer. The insulating layer may be referred to asfield oxide and the further insulating layer may be referred to as agate oxide. The insulating layer may have a greater thickness than thethickness of the further insulating layer. The insulating layer may havea greater thickness on the base of the trench than on the lower portionof the sidewalls of the trench and the thickness of the furtherinsulating layer may be greater in the region between the field plateand the base of the gate than on the sidewalls of the upper portion ofthe trench. The insulating layer may surround a gap at the base of thetrench in which the field plate may be formed. The further insulatinglayer may surround a gap at in the upper portion of the trench in whichthe gate may be formed.

The field plate may be electrically coupled to source potential by meansof a conductive redistribution structure extending directly from aportion of the field plate, or by a conductive via that is typicallypositioned outside of the active area of the semiconductor device.Alternatively, the field plate may be coupled to gate potential.

In some embodiments, the method further comprises forming a body contactregion. The body contact region includes the second conductivity typeand may be coupled with the body region. The body contact region may bemore highly doped than the body region. In embodiments in which the bodyregion and the body contact region are p-type, the body contact regionmay be formed by implanting acceptors into the semiconductor body at aposition which borders or overlaps with the body region.

In some embodiments, the transistor structure includes a gate and afield plate in different trenches. In these embodiments, the method maycomprise forming a further trench extending into the semiconductor bodyfrom the first major surface such that the further trench is laterallyspaced apart from the source region and from the trench in which thegate is to be formed. A field plate is formed in the further trench. Aninsulating layer may be first formed on the base and on the sidewalls ofthe further trench and the field plate formed on the insulating layer.The insulating layer may surround a gap in the further trench which isfilled with conductive material to form the field plate.

As discussed above, the transistor structure including the sourceregion, which comprises two different dopant species, for example afirst dopant species providing donors or acceptors and a second dopantspecies which affects, in particular, reduces or decelerates, thediffusion of the first dopant species within the semiconductor body uponapplication of heat to the semiconductor body, and which forms a pnjunction with the body region having at least a portion that is arrangedat a depth d_(pn) from the first major surface of 50 nm to 300 nm, isnot limited to that illustrated in FIG. 1 . FIGS. 3 to 14 illustratefurther examples of transistor structures in which a source regionaccording to any of the embodiments described herein may be used.

FIG. 3 illustrates a cross-sectional view of a portion of asemiconductor device 50 comprising a transistor device 51. Thesemiconductor device 50 includes a semiconductor body 52 having a firstmajor surface 53 and a second major surface 54 which opposes the firstmajor surface 53. The semiconductor body 52 may comprise silicon and maycomprise a monocrystalline epitaxial silicon layer grown on a substrateor may comprise a single crystal silicon. The transistor device 51includes a plurality of transistor structures or cells 55, 55′ coupledin parallel. Each transistor cell 55, 55′ comprises a trench 56 whichcomprises a field plate 57 arranged towards a bottom of the trench 56and a gate 58 arranged in the trench 56 above the field plate 57 and,therefore, in an upper portion of the trench 56. The trench 56 may havea substantially elongate strip-like structure extending into the planeof the drawing.

The trench 56 further includes an insulating material 59 which lines thebase 60 and sidewalls 62 of the trench 56 and electrically insulates thefield plate 57 from the gate 58. The thickness of the insulating layer59 may be greater at the base 60 and lower portion 61 of the sidewalls62 of the trench 56 such that the field plate 57 has a width which isless than a width of the gate 58. The insulating layer 59 may have asmaller thickness on the sidewalls 62 in the upper portion of the trench56 between the gate 58 and the sidewall 62 and may have a greaterthickness between an upper surface of the field plate 57 and a lowersurface of the gate 58.

The semiconductor device 50 includes a plurality of transistor cells 55which are arranged such that the trenches 56 extend substantiallyparallel to one another. The region between neighbouring trenches 56,56′ defines a mesa 65.

The semiconductor body 52 comprises a first conductivity type, which inthis particular embodiment is n-type. The transistor structure 51includes a source region 63 which comprises the first conductivity type,in this particular embodiment n-type, which extends from the first majorsurface 53 into the semiconductor body 52 at a position bordering thesidewall 62 of the trench 56. The source region 63 may form a portion ofthe sidewall 62 of the trench 56. The transistor structure 51 furtherincludes a body region 64 which comprises the second conductivity type,in this embodiment p-type, which is positioned below the source region63 with respect to the first major surface 53 and extends betweenneighbouring trenches 56, 56′.

The source region 63 forms a pn junction with the body region 64 at aninterface 67 which is positioned at a depth d_(pn) from the first majorsurface 63 of the semiconductor body 52 in regions immediately laterallyadjacent the sidewall 52 of the trench 56. This pn junction is arrangedas a depth d_(pn) from the first major surface 53 which lies in therange of 50 nm to 300 nm. The source region 63 comprises first dopantsto produce a n-type conductivity, in particular, phosphorus, and seconddopants, having a different composition from the first dopant, which areselected to affect the diffusion of the first dopant when thesemiconductor body is subjected to a thermal anneal to activate thefirst dopant and form the source region 63 of the first conductivitytype. The second dopant may include carbon and/or fluorine. The sourceregion 63 is free of arsenic dopants.

The transistor structure 51 further includes a body contact region 66which comprises the second conductivity type and which is more highlydoped than the body region 64. In the embodiment illustrated in FIG. 3 ,the body contact region 66 has the form of a well extending from thefirst major surface 53 into the semiconductor body 52. The body contactregion 66 extends between neighbouring source regions 63, 63′ arrangedat opposing sides of the mesa 65 and is positioned in a central portionof the width w_(m) of the mesa 65 and on the body region 64.

In some non-illustrated embodiments, the body contact region 66 mayreach deeper into the semiconductor body 52 than the depth of the pnjunction d_(pn) and may be located also laterally underneath a part ofthe source region 63. However, the body contact region 66 is laterallyspaced apart from the insulating layer 59 insulating the gate electrode58 from the body region 64.

The transistor device 51 further includes a drain region 68 comprisingthe first conductivity type. In this embodiment, the drain region 68 ispositioned at the second major surface 54 of the semiconductor body 62.The transistor device 51 may further include a further layer 69 that islightly doped with the first conductivity type and positioned betweenthe drain region 68 and the semiconductor body 52.

The mesa 65 has a width w_(m) which comprises, at the first majorsurface 53, the source region 63, the body contact region 66 and thesource region 63′ of the neighbouring transistor cell 55. The mesa 65has a length l_(m) which extends substantially perpendicular to thewidth w_(m) into the plane of the drawing. The height of the mesacorresponds to the depth of the trench d_(t). The body region 64 extendsacross the entire width w_(m) of the mesa 65.

FIG. 4 illustrates a cross-sectional view of a portion of asemiconductor device 70 including a transistor structure 71 which issimilar to that illustrated in FIG. 3 , whereby like reference numeralsdenote similar parts. The transistor structure 71 differs in thearrangement of the source region 63 and body contact region 66.

FIG. 4 illustrates two trenches 56, 56′ separated by a mesa 65, wherebyeach trench 56 includes a field plate 57 arranged towards the bottom ofthe trench 56 and a gate 58 arranged towards the top of the trench 56which is electrically insulated from the field plate 57. In thisembodiment, a single source region 63 extends the across the top surfaceof the mesa 65 and extends across the entire width w_(m) of the mesa 65.The single source region 63 forms part of the sidewall 62 of the twoneighbouring trenches 56, 56′. The body region 64 also extends acrossthe entire width w_(m) of the mesa 65 and is positioned underneath andforms a pn junction 67 with the source region 63 which extendsthroughout the entire width w_(m) of the mesa 65. The pn junction 67 isarranged at a depth d_(pn) of between 50 nm and 300 nm from the firstmajor surface 53.

In this embodiment, the transistor structure 71 includes a body contactregion 66 which is arranged adjacent the source region 63 in thedirection of the length l_(m) of the mesa 65. The body contact region 66is arranged on a portion of the body region 64 which is arrangedadjacent the source region 63 in the direction of the length l_(m) ofthe Mesa 65. The body contact region 66 also extends across the entirewidth w_(m) of the mesa 65 and forms a pn junction with the sourceregion 63 that extends across the entire width w_(m) of the mesa 65. Thetop surface of the mesa 65 comprises alternate source regions 63 andbody contact regions 66 in a direction l_(m) which is substantiallyperpendicular to the width w_(m) of the mesa 65 and to the depth d_(t)of the trenches 56, 56′.

FIG. 5 illustrates a cross-sectional view of a portion of asemiconductor device 80 including a transistor structure 81. Thetransistor structure 81 differs from that illustrated in FIG. 3 in thatthe mesa 65 includes a shallow trench 82 which extends from the firstmajor surface 53 into the semiconductor body 52 and has a depth d_(ts)to which is less than the depth d_(t) of the trenches 56 which includethe field plate 57 and gate 58. The mesa 65 includes a source region 63which extends from the sidewall 62 of the trench 56 to a sidewall 84 ofthe shallow trench 82 and from the sidewall 62′ of the trench 56′ to thesidewall 85 of the shallow trench 82. The source region 63 borders theupper portion of the sidewalls 84, 85 shallow trench 82 and is arrangedon a body region 64 which extends across the width w_(m) of the mesa 65.The base 83 of the shallow trench 82 is arranged in the body region 64.The lower part of the sidewalls 84, 85 of the shallow trench 82 isformed by the body region 64. The depth d_(ts) of the base of theshallow trench 82 from the first major surface 53 is greater than thedepth d_(pn) of the interface between the source region 63 and the bodyregion 64. The transistor structure 81 further includes a body contactregion 66 which is arranged in the body region 64 and surrounds the base83 of the shallow trench 82.

In some non-illustrated embodiments, the body contact region 66 maylaterally exceed the dimensions of the shallow trench 82. However, thebody contact region 66 will be laterally spaced apart from theinsulating layer 59 insulating the gate electrode 58 from the bodyregion 64. The shallow trench 82 may be lined or filled with conductivematerial and may be coupled to source potential.

FIG. 6 illustrates a cross-sectional view of a portion of asemiconductor device 90 including a transistor structure 91 andillustrates two trenches 56, 56′ separated by a mesa 65. In thisembodiment, the mesa 65 includes a shallow trench 82 which extends fromthe first major surface 53 into the body region 64. The source region 63is arranged on the body region and bounds sidewalls 84, 85 of theshallow trench 82 as in the embodiment illustrated in FIG. 5 .

The trenches 56 include a field plate 57 and gate 58 as in theembodiments illustrated in FIGS. 3 to 5 . However, in the embodimentillustrated in FIG. 6 , the gate 58 has a split gate structure and thefield plate 57 extends to the first major surface 53. The lower portion92 of the field plate 57 is laterally wider than an upper portion 93 ofthe field plate 57. The upper portion 93 extends to the first majorsurface 53. The gate 58 includes two portions 94 which are arranged onopposing sides of the narrow upper portion 93 of the field plate 57. Thetwo portions 94 of the gate 58 are electrically insulated from the fieldplate 93 by an electrically insulating layer 95. The thickness of theelectrically insulating layer 95 may be greater at the sidewalls 62 andbase 60 of the trench 56 to provide a field oxide (Fox) and have asmaller thickness between the portions 94 of the gate and sidewall ofthe trench 56 to provide a gate oxide (Gox). The thickness of theinsulating layer 95 between the shoulders of the field plate 57 and thegate portion 94 lying above the shoulders may be greater than thethickness of the insulating layer 95 providing the gate oxide. Theinsulating layer 95 may be provided by two or more sublayers.

In the transistor structures illustrated in FIGS. 3 to 6 , the gate andthe field plate are arranged in the same trench. However, in otherembodiments, the gate and the field plate may be arranged in differenttrenches.

FIG. 7 illustrates a semiconductor device 100 comprising a transistordevice 101 in which the gate 102 is positioned in a first trench 103 andthe field plate 104 is arranged in a second trench 105 that is laterallyspaced apart from the first trench 103. The first trench 103 and secondtrench 105 extend from the first major surface 106 into thesemiconductor body 107. The semiconductor body 107 may be considered tocomprise a mesa 108 formed by and extending between the second trenches105. The second trench 105 has a depth d_(t2) which is greater than thedepth d_(t1) of the first trench 103. The first trench 103 and secondtrench 105 have an elongate strip-like form extending into the plane ofthe drawing. The base 116 of the second trench 105 is arranged withinthe semiconductor body 107 and the base 117 of the first trench 103 isarranged in the mesa 108 defined by the second trenches 105. Thesemiconductor body 107 may be doped with the first conductivity type.The semiconductor body 107 may be provided by an epitaxial silicon layerarranged on a substrate, for example a heavily doped substrate of thesecond conductivity type.

The second trench 105 is lined with an insulating material forming aninsulating layer 109 which surrounds the field plate 104 andelectrically insulating the field plate 104 from the semiconductor body107. The first trench 103 is also lined by insulating layer 110 whichsurrounds the gate 102. The thickness of the insulating layer 109 may begreater at the base of the first trench 103 in comparison to itsthickness between the sidewalls of the gate 102 and the sidewalls 118 ofthe first trench 103. Optionally, the thickness of the insulating layer109 may substantially equal at the base of the first trench 103 incomparison to the thickness of the insulating layer 110 between thesidewalls of the gate 102 and the sidewalls 118 of the first trench 103.The transistor structure 101 includes source regions 111 comprising twodifferent dopant species which have the form of a well formed in thefirst surface 106 adjacent opposing sidewalls 110 of the first trench103. The source regions 111 form part of the sidewall 118 of the firsttrench 103. The source regions 111 are formed in a body region 112 whichextends between the sidewall 119 of the second trench 105 and thesidewall 118 of the first trench 103 and between the opposing sidewall118′ of the first trench 103 and the sidewall 119′ of the second trench105′. The body region 112 forms a pn junction with the underlyingportion of the semiconductor body 107 forming the mesa 108. A pnjunction 113 is formed between the source region 111 and the body region112 which has a depth d_(pn) from the first major surface 106 at itsdeepest point which lies within 50 nm and 300 nm of the first majorsurface 106.

The source region 111 comprises two different dopant species; firstdopants to produce a n-type conductivity, in particular, phosphorus, andsecond dopants, having a different composition from the first dopant,which are selected to affect the diffusion of the first dopant when thesemiconductor body is subjected to a thermal anneal to activate thefirst dopant and form the source region 111 of the first conductivitytype. The second dopant may include carbon and/or fluorine. The sourceregion 111 is free of arsenic dopants.

A conductive layer 114, which may include metal, is arranged on each ofthe second trenches 105 and may be directly in contact with theconductive field plate 104 which extends to the first major surface 106.The conductive layer 114 may also extend onto and be in direct contactwith the source region 111 and the body region 112 and be electricallyinsulated from and separate from the gate 102 in the first trench 103.The transistor structure 101 may also include a gate metal 115 which isarranged on the gate 102 and is electrically insulated from theconductive layer 114 that is coupled to the source region 111.

FIG. 8 illustrates a semiconductor device 120 comprising a transistordevice 121 which has a similar transistor structure to that illustratedin FIG. 7 . Like reference numerals are used to indicate similar parts.

The transistor structure 121 differs from the transistor structure 101illustrated in FIG. 7 in the arrangement of the gate 102 within thefirst trench 103. The transistor structure 121 includes second trenches105, 105′ defining a mesa 108 in which the first trench 103 includingthe gate 102 is positioned. The source region 111 and the body region112 are arranged on opposing sides of the first trench 103 as in thetransistor structure 101 illustrated in FIG. 7 .

The first trench 103 includes an insulating layer 110 which lines thebase 117 and sidewalls 118 of the first trench 103 and forms a gap inthe first trench 103 in which the conductive material forming the gate102 is positioned. In the semiconductor device 120, the upper surface ofthe gate 102 is spaced at a distance from the first major surface 106 ofthe semiconductor body 107 and is positioned within the first trench103. The gate 102 is covered by an insulating layer 110 which extends tothe first major surface 106 and is coplanar with the first major surface106. In this embodiment, the gate 102 has an upper surface which ispositioned below the first major surface 106 of the semiconductor bodyand below the uppermost surface of the mesa 108 such that the uppermostportion of the gate 102 overlaps with the lower portion of the sourceregion 111. In this embodiment, the conductive layer 114, which iselectrically coupled to the field plates 104 in the second trenches 105,the body region 112 and the source region 111, also extends directlyover the top of the first trenches 103 as it is electrically isolatedfrom the gate 102 in the first trenches 103 by the insulating layer 110arranged on the gate 102. An electrical connection to the gate 102 maybe formed by a conductive via (not seen in the view of FIG. 8 ) which ispositioned laterally outside and spaced apart from the conductive metallayer 114. The conductive via may be arranged in an edge region, forexample, and be electrically coupled to a metal layer which may bereferred to as a gate runner. The metal layer may extend substantiallyperpendicular to the plurality of first trenches 103 and electricalcouple the gates 102 arranged in the plurality of first trenches 103together.

FIG. 9 illustrates a semiconductor device 130 including a transistorstructure 131 which is similar to that illustrated in FIG. 7 , wherebylike reference numerals indicate like parts. The transistor structure131 differs from the transistor structure 101 illustrated in FIG. 7 inthe arrangement of the source region 111.

In the embodiment illustrated in FIG. 9 , the source region 111 extendsfrom the sidewall 118 of the first trench 103 to the sidewall 119 of thesecond trench 105 and forms a portion of the sidewall 118 of the firsttrench 103 and sidewall 119 of the second trench 105. The transistorstructure 131 may also include a body contact region 132 which extendsfrom the source region 111 into the body region 112. The body contactregion 132 may be positioned adjacent the sidewall 119 of the secondtrench 105 and may be positioned in the body region 112. The bodycontact region 132 has a lateral extent which is smaller than thedistance between facing sidewalls 118, 119 of the first trench 103 andsecond trench 105, respectively, such that the source region 111 forms apn junction with the body region 112 in the region immediately adjacentthe sidewall 118 of the first trench 103 and a pn junction with the bodycontact region 132 adjacent the sidewall 119 of the second trench 105.In the embodiment illustrated in FIG. 9 , the conductive metal layer 114extends into the upper portion of the second trench 105 and is in directcontact with the field plate 104, the body contact region 132, the sideface and top face of the source region 111.

The gate 102 may be substantially coplanar with the first major surface106 and may be covered by an insulating layer 115 which protrudes abovethe first major surface 106 of the semiconductor body 107. The metallayer 114 may be arranged on this insulating layer 115.

In a further embodiment (not shown in FIG. 9 ), the gate 102 has anupper surface which is positioned below the first major surface 106 ofthe semiconductor body and below the uppermost surface of the mesa 108such that the uppermost portion of the gate 102 overlaps with the lowerportion of the source region 111. This embodiment combines the featuresfor connecting the field plates 104, source regions 111 and body regions112 as described with respect to FIG. 9 with the features for the gateelectrode 102 with respect to the source region 111 as described in FIG.8 .

FIG. 10 illustrates a semiconductor device 140 including a transistorstructure 141 which is similar to that illustrated in FIG. 9 . Thetransistor structure 141 differs from the transistor structure 131 inthe arrangement of the gate 102 with respect to the first major surface106. In this embodiment, the uppermost surface of the gate 102 ispositioned above the uppermost surface of the source region 111.

In the embodiments illustrated in FIGS. 1 to 10 , the trenches have anelongate or strip-like structure such that they extend substantiallyparallel to one another into the plane of the drawing. The trenches mayinclude a gate and a field plate, just a gate or just a field plate.However, the trenches may have forms other than elongate or strip-likeforms in plan view and may be have the form of columnar trenches. Acolumnar trench may have various forms in plan view. For example, thecolumnar trench may be circular, square, or hexagonal in plan view. Inembodiments in which the trenches have a columnar structure, thetrenches may be arranged in a two-dimensional array in plan view, forexample in rows and columns which may be aligned, for example to form asquare grid array or a rectangular grid array, or adjacent rows may beoffset, for example to produce a hexagonal close packed type array.

The columnar trenches may include only a field plate and separateelongate strip-like trenches may be provided for the gate. In someembodiments, rows of columnar trenches including a field plate may bearranged on either side of an elongate trench including a gate. Thecolumnar trenches including the field plate may be spaced at intervalson either side of the strip-like elongate trench including thestrip-like elongate gate. In some embodiments, a trench having the formof a grid may be provided for the gate and columnar trenches including afield plate may be arranged on either side of the trench including agate such that a columnar trench is arranged in each cell defined by thegrid of the gate trench.

FIGS. 11 to 13 illustrate examples of semiconductor devices including atransistor structure which has elongate trenches for the gate andcolumnar type trenches for the field plates. For better clarity, thesource and gate metals providing electrical contact to the outside havebeen omitted. In the FIGS. 11 to 13 , Source metal will be available ontop of the first major surface 153 and providing low ohmic contact thedoped semiconductor regions reaching an opened surface.

FIG. 11 illustrates a semiconductor device 150 comprising a transistorstructure 151. The semiconductor 150 comprises a semiconductor body 152including a first major surface 153 and a second major surface 154opposing the first major surface 153. The transistor structure 151includes a first trench 155 which extends from the first major surface153 into the semiconductor body 152. The first trench 155 has anelongate form and in plan view the first trench 155 has the form of agrid. A gate electrode 156 is arranged in the first trench 155 and hasthe form of a grid. The gate 156 is electrically conductive and maycomprise poly silicon or metal. An insulating layer 157 lines thesidewalls and base of the first trench 155 and electrically insulatesthe gate 156 from the semiconductor body 152. The upper surface of thegate 156 may be covered by an insulating layer which protrudes above thefirst surface first major surface 153 of the semiconductor body 152.Alternatively, if the upper surface of the gate 156 lies in a planebelow the plane of the first major surface 153, the insulating layercovering the upper surface of the gate may be substantially coplanarwith the first major surface 153.

The first trench 155 defines a plurality of mesas 159 protruding fromthe semiconductor body 152. In embodiments in which the first trench 155forms a square grid, each of the mesas 159 may be substantially squarein plan view. The transistor structure 151 further includes a pluralityof second trenches 160 which have a columnar form and are arranged ineach of the mesas 159 and extend from the first major surface 153 intothe semiconductor body 152. The second trenches 160 include a fieldplate 161 which also has a columnar structure. The second trench 160includes an insulating layer 162 which lines the sidewalls and base ofthe trench 160 and electrically insulates the field plate 161 from thesemiconductor body 152. The field plate 161 may have lateral area whichis substantially the same throughout its height. In some embodiments,the field plate 162 and/or the trench 160 may taper towards the base toprovide a specular or needle-like form.

The mesa 159 includes a source region 163 arranged on a body region 164.The mesa 159 may also include a body contact region 165 which is incontact with both the body region 164 and the source region 163.

The semiconductor body 152 may include the first conductivity type. Thesource region 163 also includes the first conductivity type and is morehighly doped than the body region 152. The body region 164 and the bodycontact region 165 include the second conductivity type. The bodycontact region 165 is more highly doped than the body region 164.

The transistor structure 151 further includes a drain region, whichcomprises the first conductivity type and which is more highly dopedthan the semiconductor body 152. The drain region is arranged underneaththe trench 160 such that it is spaced apart from the base of the secondtrench 160 by a portion of the semiconductor body 152. The drain regionmay form the second major surface 154 or may be positioned between thesemiconductor body 152 and a further substrate.

The source region 163 forms a pn junction with the body region 164 whichis positioned at a depth d_(pn) from the first major surface 153 of thesemiconductor body 152. This pn junction is arranged as a depth d_(pn)from the first major surface 153 which lies in the range of 50 nm to 300nm. The source region 163 comprises first dopants to produce a n-typeconductivity, in particular, phosphorus, and second dopants, having adifferent composition from the first dopant, which are selected toaffect the diffusion of the first dopant when the semiconductor body issubjected to a thermal anneal to activate the first dopant and form thesource region 163 of the first conductivity type. The second dopant mayinclude carbon and/or fluorine. The source region 163 is free of arsenicdopants.

The arrangement of the source region 163, body region 164 and bodycontact region 165 may vary. FIGS. 11 to 13 illustrate three examples.

In the transistor structure 151 illustrated in FIG. 11 , the sourceregion 163 is arranged immediately adjacent the first trench 157 and thegate 156. The body contact region 165 is arranged adjacent the sidewall167 of the second trench 160 and adjacent the source region 163 suchthat it forms a substantially vertical pn junction with the sourceregion 163. Both the source region 163 and the body contact region 165are arranged on the body region 164 which extends from the first trench157 to the second trench 160. In this embodiment shown in FIG. 11 , thebody contact region 165 and the source region 163 have substantiallyequal depth into the semiconductor body 152. As described in combinationwith FIGS. 3 and 5 , the body contact region 165 may also reach deeperinto the semiconductor body 152.

In the embodiment illustrated in FIG. 12 , the source region 163 extendsfrom the sidewall 168 of the first trench 157 including the gate 156 tothe sidewall 167 of the second trench 160 including the field plate 161.The body contact region 165 is arranged in the body region 164 adjacentthe sidewall 167 of the second trench 160 and is in contact on its uppersurface with the source region 163 and is surrounded on its lower sideby the body region 164. The body contact region 165 can be considered tobe arranged in the body region 164. In this embodiment, the field plate161 overlaps with the body region 164 and body contact region 165 andthe upper surface of the field plate 161 is arranged in the trench in aplane below the plane of the first major surface 153 and below theinterface between the source region 163 and the body region 164.

The embodiment illustrated in FIG. 13 , is similar to that described inFIG. 12 . However, the field plate 161 is recessed deeper into thesemiconductor body 152 than the upper end of the insulating layer 162.The field plate 161 in this embodiment is recessed less into the depthof the semiconductor body 152 than the depth of the body region. Theembodiment according to FIG. 13 may provide more process flexibilityconcerning the recess of the field plate 161 and the ohmic contact ofsource region 163 and body contact region 165 to the source metal (notshown in FIGS. 11 to 13 ) as compared to FIG. 12 .

FIG. 14 illustrates a partial cross-sectional view of a portion ofsemiconductor device 170 including a transistor structure 171. Thetransistor structure 171 includes a trench 172 which extends from afirst major surface 173 of a semiconductor body 174, which comprises thefirst conductivity type, into the semiconductor body 174. The trench 172includes a gate 175 arranged in its upper portion which extends into afield plate 176 in the lower portion of the trench 172. The field plate176 has a lateral width w_(f) which a smaller than the lateral widthw_(g) of the gate 175. The trench 172 includes an insulating layer 177which lines the base and sidewalls of the lower portion of the trench172 and provides a field oxide (Fox). The trench 172 includes a furtherinsulating layer 178 which lines the sidewalls of the upper portion ofthe trench 172 and has a thickness which is less than the thickness ofthe insulating layer 177 in the lower portion of the trench 172. Thefurther insulating layer 178 may provide a gate oxide (Gox). The gate175 with the integrated field plate 176 may be covered on its uppersurface by further insulating layer 179 which may protrude above thefirst major surface 173 of the semiconductor body 174. The trench 172may be elongate and have a strip-like form in plan view.

The semiconductor body 174 includes a mesa portion 180 extending onopposing sides of the trench 172. Shallow trenches 181 are arranged onopposing sides of the trench 172 forming a mesa 182 in which the trench172 is arranged and bounded by the mesa portions 180. The mesa 182includes a source region 183 comprising the first conductivity type,which is more highly doped than the semiconductor body 174 which isarranged in a strip-like well adjacent opposing sides of the trench 172.The source region 183 is arranged on a body region 184 which extendsunder the base of the shallow trench 181. The transistor structure 171further includes a body contact region 185 which is arranged at andforms the base of the shallow trench 181 and which is surrounded by thebody region 184.

In the embodiments described herein, the source region forms a pnjunction with the underlying body region which is positioned at adistance or depth from the first major surface of the semiconductorbody. At least a portion of the pn junction or interface is positionedat a distance of between 50 nm to 300 nm from the first major surface.The first major surface may be formed by upper portion of the sourceregion such that this depth corresponds to the depth of the sourceregion. The source region includes at least two types of dopant, wherebyone or more of the dopants comprises a donor or acceptor to provide thedesired conductivity type. In the illustrated embodiments, the sourceregion includes phosphorus as a donor in order to provide n-typeconductivity. The source regions also include a further dopant, which isdifferent from the dopant providing the conductivity type, which isprovided to provide a mechanism for controlling the diffusion of thefirst dopant or dopants during thermal annealing. Depending on thediffusion mechanism of the particular first dopant, the second dopantmay be selected to prevent diffusion using a different mechanism. Forexample, diffusion of phosphorous ions may be controlled by carbonand/or fluorine dopants.

Discrete power switches with rather low blocking voltages (e. g. up to300V, 100V, 60V, 30V) may have the requirement to provide ultra-lowsource path resistances since the total on-state resistance of thesedevices is already very low. Accordingly, any parasitic seriesresistances should be minimized. The resistance of the source isnegative since it not only leads to a voltage drop in the load path butalso to a reduced effective gate-channel-voltage drop reducing thechannel conductivity and increasing turn-on-losses. On the other hand,the length of the source doping region to the channel region is largecompared to state-of-the art CMOS devices and may reach values up to 200nm or more due to constructive reasons of the transistor cell.Additionally, the source doping should not reach deeper into thesemiconductor body than to the channel region to minimize the channellength/channel resistance and to avoid premature punch-through of theelectric field in blocking operation.

Whilst high dose As implantation can be used to form shallow profiles,there is a risk of vacancy defect formation from As precipitation whichpotentially causes drain-source leakage currents, resulting in chipfailure and low production yields.

According to embodiments described herein, the high dose As Implantationis replaced by a combination of Carbon Implantation and PhosphorousImplantation in order to achieve the same (or even better) electricalcharacteristics while reducing to even eliminating the risk of leakagecurrent fails during blocking state at blocking voltages approaching thenominal blocking voltage of the device due to the creation of Asdefects. Some implanted dopants, such as carbon and fluorine reducephosphorus diffusion. This can be used to produce to source-profileswith a very similar vertical extension to conventional As-profiles.

Phosphorus transient enhance diffusion is thought to occur via aninterstitial diffusion mechanism. Techniques to block this mechanism andto control it include a carbon kick-out mechanism and fluorine-vacancyclusters, both of which can be used to consume the interstitials drivingdiffusion of the phosphorous. Additionally, P can be activated moreefficiently in Si than As. Therefore, either better conductivity at thesame implantation dose or a reduction of the implantation dose (andprocess cost) may be used to achieve the same electrical performance asthat achieved by As.

The dimensions in power transistors differ from those of advanced logicsince the source region has to penetrate deeper into the semiconductorto form an electric connection to the (mainly vertical) inversionchannel at the gate dielectric. It is therefore not the target toestablish ultra-shallow source regions but deeper source regions with apn-junction at e.g. 50 nm to 200 nm from the Si surface whilstmaintaining the same requirements for a steep gradient for low ohmicchannel connection at a defined channel length.

In order to optimize a plateau-like P doping profile a sequence of Cand/or F implantations with different implantation energies can be used.Channeling implantation conditions as well as the use of an energydiffusor in front of the wafer, transforming the mono-energetic ion beaminto a multi-energy one—yielding an adjustable ion depth distribution inthe semiconductor device—may be used. Thereby, diffusion can beregulated according to device needs, e.g. an optimum verticaldistribution of P in the source zone. For the case of steepestgradients, a Co-Implantation of Si and C before applying the P dopingimplant may be beneficial.

Different sequences of C only, F only or a combined sequence of C and Fimplantation can be used, followed by a single or multiple Pimplantations. Ge could be used.

Besides optimized source regions, device regions deep in thesemiconductor body can be tailored, e.g. ˜10 μm plateau-like P dopingprofiles with a steep gradient at the end of the plateau. This can berealized either by the use of an energy diffusor for C/F/P implantationsor by a sequence of implantation energies of these elements.

In the embodiments describe above, the semiconductor body may have adoping concentration which is usefully in the range of 1.10¹⁴ cm⁻³ to1.10¹⁸ cm⁻³, conveniently in the range of 7.10¹⁴ cm⁻³ to 3.10¹⁷ cm⁻³,and more conveniently in the range of 1.10¹⁵ cm⁻³ to 1.10¹⁶ cm⁻³, buthigher or lower doping concentrations may also be used. The drain regionmay have doping concentration usefully at least 5.10¹⁹ cm⁻³,conveniently at least 1.10²⁰ cm⁻³ and more conveniently at least 3.10²⁰cm⁻³, but higher or lower doping concentrations may also be used. Thesource region may have doping concentration usefully of at least 5.10¹⁹cm⁻³, conveniently of at least 1.10²⁰ cm⁻³, and more conveniently of atleast 3.10²⁰ cm⁻³, but higher or lower doping concentrations may also beused. The body region may have doping concentration usefully in therange of 1.10¹⁷ cm⁻³ to 1.10²⁰ cm⁻³, conveniently in the range of 2.10¹⁷cm⁻³ to 7.10¹⁸ cm⁻³, and more conveniently in the range of 5.10¹⁷ cm⁻³to 5.10¹⁸ cm⁻³, but higher or lower doping concentrations may also beused. The body contact region may have a doping concentration in therange of 1.10¹⁸ cm⁻³ to 2.10²⁰ cm⁻³.

Spatially relative terms such as “under”, “below”, “lower”, “over”,“upper” and the like are used for ease of description to explain thepositioning of one element relative to a second element. These terms areintended to encompass different orientations of the device in additionto different orientations than those depicted in the figures. Further,terms such as “first”, “second”, and the like, are also used to describevarious elements, regions, sections, etc. and are also not intended tobe limiting. Like terms refer to like elements throughout thedescription.

As used herein, the terms “having”, “containing”, “including”,“comprising” and the like are open ended terms that indicate thepresence of stated elements or features, but do not preclude additionalelements or features. The articles “a”, “an” and “the” are intended toinclude the plural as well as the singular, unless the context clearlyindicates otherwise. It is to be understood that the features of thevarious embodiments described herein may be combined with each other,unless specifically noted otherwise.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. A method for fabricating a semiconductor device,the method comprising: forming a trench in a first major surface of asilicon semiconductor body comprising a first conductivity type; forminga gate in the trench; forming a body region of a second conductivitytype in the silicon semiconductor body; implanting a second dopantspecies into a first region of the body region; implanting a firstdopant species into a second region of the body region, wherein thefirst dopant species provides the first conductivity type and whereinthe second dopant species is different from the first dopant species andreduces diffusion of the first dopant species in the siliconsemiconductor body; and thermally annealing the silicon semiconductorbody to form a source region in the silicon semiconductor body, thesource region comprising the first dopant species and the second dopantspecies, and to produce a pn-junction between the source region and thebody region at a depth d_(pn) from the first major surface, wherein 50nm<d_(pn)<300 nm, wherein the first dopant species comprises phosphorousions, wherein the second dopant species comprises carbon and/orfluorine.
 2. The method of claim 1, wherein the thermal annealing iscarried out at a temperature in a range of 900° C. to 1050° C. for atime in a range of 1 second to 360 seconds.
 3. The method of claim 1,wherein the first dopant species and the second dopant species areimplanted sequentially into the body region.
 4. The method of claim 1,wherein the first dopant species and the second dopant species are eachimplanted in multiple steps at decreasing or increasing depths from thefirst major surface.
 5. The method of claim 1, further comprising:forming a field plate in a base of the trench; and forming the gate onthe field plate.
 6. The method of claim 1, further comprising: forming afurther trench extending into the first major surface, the furthertrench being laterally spaced apart from the source region; and forminga field plate in the further trench.
 7. The method of claim 1, furthercomprising: forming a drain region of the first conductivity type in thesilicon semiconductor body under the trench.
 8. The method of claim 1,wherein the source region comprises a phosphorous concentration in arange of 5.10¹⁹ atoms/cm³ to 7.10²⁰ atoms/cm³ and a carbon concentrationin a range of 1.10¹⁹ atoms/cm³ to 7.10²⁰ atoms/cm³.
 9. The method ofclaim 1, further comprising: forming a body contact region of the secondconductivity type, wherein the body contact region is more highly dopedthan the body region.
 10. The method of claim 9, wherein the trench iselongated and has a depth, a width and a length, the length beinggreater than the width, and wherein the body contact region is arrangedadjacent the source region in a direction of the width of the trench orin a direction of the length of the trench.
 11. The method of claim 10,further comprising: forming a shallow trench extending from the firstmajor surface, wherein sidewalls of the shallow trench are formed by thesource region and the body region, and wherein the body contact regionforms a base of the shallow trench.
 12. The method of claim 1, whereinthe second dopant species comprises only carbon.
 13. The method of claim1, wherein the second dopant species comprises only fluorine.
 14. Themethod of claim 1, wherein the second dopant species comprises bothcarbon and fluorine.
 15. The method of claim 1, wherein the seconddopant species further comprises germanium.
 16. The method of claim 1,further comprising: forming a device region of the first conductivitytype formed deeper in the silicon semiconductor body than the sourceregion, wherein the device region comprises the first dopant speciesproviding the first conductivity type and the second dopant speciesreducing the diffusion of the first dopant species in the siliconsemiconductor body.
 17. A method for fabricating a semiconductor device,the method comprising: forming a trench in a first major surface of asemiconductor body comprising a first conductivity type; forming a gatein the trench; forming a body region of a second conductivity type inthe semiconductor body; implanting a second dopant species into a firstregion of the body region; implanting a first dopant species into asecond region of the body region, wherein the first dopant speciesprovides the first conductivity type and wherein the second dopantspecies is different from the first dopant species and reduces diffusionof the first dopant species in the semiconductor body; and thermallyannealing the semiconductor body to form a source region, the sourceregion comprising the first dopant species and the second dopantspecies, and to produce a pn-junction between the source region and thebody region at a depth d_(pn) from the first major surface, wherein 50nm<d_(pn)<300 nm, wherein the first dopant species and the second dopantspecies are each implanted in multiple steps at decreasing or increasingdepths from the first major surface.
 18. A method for fabricating asemiconductor device, the method comprising: forming a trench in a firstmajor surface of a semiconductor body comprising a first conductivitytype; forming a gate in the trench; forming a body region of a secondconductivity type in the semiconductor body; implanting a second dopantspecies into a first region of the body region; implanting a firstdopant species into a second region of the body region, wherein thefirst dopant species provides the first conductivity type and whereinthe second dopant species is different from the first dopant species andreduces diffusion of the first dopant species in the semiconductor body;thermally annealing the semiconductor body to form a source region, thesource region comprising the first dopant species and the second dopantspecies, and to produce a pn-junction between the source region and thebody region at a depth d_(pn) from the first major surface, wherein 50nm<d_(pn)<300 nm; and forming a device region of the first conductivitytype formed deeper in the semiconductor body than the source region,wherein the device region comprises the first dopant species providingthe first conductivity type and the second dopant species reducing thediffusion of the first dopant species in the semiconductor body.